The present invention relates to a crash sensor adapted for installation on an automotive vehicle equipped with a passive occupant restraint device such as an inflatable air bag or seat belt tensioner. When such a vehicle is subjected to deceleration of the kind accompanying a crash, and the crash sensor triggers, the air bag is inflated to provide a protective cushion for the occupant or the seat belt is pulled back against the occupant holding him in a safe position.
Gas damped crash sensors have become widely adopted by many of the world's automobile manufactures for sensing a crash and for initiating the inflation of an air bag or tensioning of seat belts. Sensors constructed from a ball and a tube are disclosed in the U.S. Pat. Nos. 3,974,350, 4,198,864, 4,284,863, 4,329,549, 4,573,706 and 4,900,880 to D. S. Breed. A sensor constructed in the form of a rod with an attached coaxial disk, both arranged to move within a cylinder, is disclosed in the U.S. Pat. No. 4,536,629 to R. W. Diller.
Recently, it has been found that although the sensors disclosed in the Breed patents generally perform well during high speed crashes, their performance deteriorates significantly in marginal crashes, especially when strong cross-axis accelerations are present. One automobile manufacturer requires that the air bag always be deployed during crashes into barriers at 12 mph or above, while not deploying the air bag in crashes into barriers at 9 mph or below. Crash sensors that are designed to meet this criterion, perform well on laboratory shock test equipment. However, when placed on a vehicle and crash tested into a barrier at 12 mph, the sensor frequently either does not trigger at all or it triggers late. In the first case the occupant does not receive the protection of the air bag or belt tensioning device, and in the second case the occupant, who is out of position, is at risk of being injured by the deployment of the air bag.
It has been hypothesized and shown theoretically that there are some conditions in which the sensing ball does not merely roll down one side of the tube but in fact undergoes a rather complicated whirling or orbiting motion. When this happens, a significant amount of energy is dissipated through sliding friction between the ball and the tube. This phenomenon has the effect of substantially delaying the motion of the ball and, on a marginal crash, can lead to a no-trigger or a late trigger condition. A similar condition has been found to exist in sensors having a cylindrical sensing mass traveling in a tube.
Deviations from linear motion are caused by accelerations perpendicular to the longitudinal axis of the sensor tube. In the typical mounting arrangement, the sensor tube axis points toward the front of the vehicle and it is the accelerations in the vertical and lateral directions that can cause the whirling motion described above.
This cross-axis effect is determined, in part, by the friction between the ball and its surrounding cylinder and thus the effect can be substantially reduced by lowering the coefficient of friction through the use of a low friction coating on the ball and/or cylinder surface.
Cross-axis vibrations have other undesirable effects, particularly on the electrical contact design currently used in gas damped ball-in-tube sensors. In particular, since the standard contact is a cantilevered beam, vibrations of the sensor can cause the contacts to vibrate and result in several intermittent "tic" closures before solid contact is achieved. Similarly, when the contacts are first impacted by the sensing mass (i.e. the ball, in the case of the ball-in-tube sensor), they frequently bounce one or more times. In one particular test crash at 14 mph in which significant cross-axis accelerations were present, the ball momentarily bridged the contacts causing a "tic" closure of insufficient duration to reliably trigger the air bag. Although this closure was on time, the air bag was not enabled until much later, once a more solid contact closure had been formed.
The ball-in-tube sensor currently in widespread use has a magnetic bias. Both ceramic and Alnico magnets are used depending upon the amount of variation in bias force, caused by temperature, that can be tolerated. Sensors used in the crush zone of the vehicle, and safing or arming sensors used both in the crush zone and out of the crush zone, can have ceramic magnets since they can tolerate a wide variation in bias force. Alnico magnets are used for the higher biased non-crush zone discriminating sensors where little variation in the bias can be tolerated. If a spring bias is employed in place of the magnetic bias as shown in the U.S. Pat. No. 4,580,810 to T. Thuen, the variation of the bias force with temperature can be practically eliminated. The use of a spring bias can also have the effect of reducing contact bounce and minimizing the effect of cross-axis vibration on the contacts. The U.S. Pat. No. 4,536,629 to R. W. Diller discloses a rod-in-cylinder gas damped crash sensor in which a contact spring is employed to provide a spring bias to the sensing mass. The U.S. Pat. No. 4,116,132 to Bell also uses a spring for bias. These sensors are also susceptible to contact bounce during operation.
The U.S. Pat. No. 4,900,880 to D. S. Breed discloses a spring biased sensor where one contact is used as the biasing spring. Although this design is suitable for some applications, particularly where the travel of the mass is relatively short, a single cantilever spring either becomes excessively long or exhibits a substantial force variation for longer travel sensors such as are currently used in the crush zone locations. Other types of springs such as coil springs, add undesirable frictional forces which deteriorate the sensor performance, especially in the presence of cross-axis vibrations. Also, when the second more rigid contact is flexible, provision must be made to prevent early closure due to vibrational excitations of this contact spring. Ball-in-tube sensors as described in the above referenced patents, and as currently manufactured, exhibit wide manufacturing tolerances due in part to the difficulty in maintaining the precise clearance between the ball and tube. Some means of adjustment or calibration during manufacture is therefore desirable. U.S. Pat. No. 4,116,132 to Bell, shows an adjustment system for application to a band and roller sensor. The same principle of a screw to fix the initial position of the sensing mass can of course be applied to the ball-in-tube as well as other sensors. Such systems require adjustment of the sensor at an early manufacturing stage before final assembly. It would be desirable if such adjustment could take place during the final sensor testing phase.
Some automobile manufacturers have a requirement that crash sensors be testable. At some time, usually during the start up sequence, an electronic circuit sends a signal to the sensor to close and determines that the contacts did close. In this manner, the sensor is operated and tested that it is functional.